ly upon the connection at _S_, which is
alternately made and broken, and in this way produces sound.

[Illustration: FIG. 216.--Push button.]

The sign "Bell out of order" is usually due to the fact that the
battery is either temporarily or permanently exhausted. In warm
weather the liquid in the cell may dry up and cause stoppage of the
current. If fresh liquid is poured into the vessel so that the
chemical action of the acid on the zinc is renewed, the current again
flows. Another explanation of an out-of-order bell is that the liquid
may have eaten up all the zinc; if this is the case, the insertion of
a fresh strip of zinc will remove the difficulty and the current will
flow. If dry cells are used, there is no remedy except in the purchase
of new cells.

299. How Electricity may be lost to Use. In the electric bell, we
saw that an air gap at the push button stopped the flow of
electricity. If we cut the wire connecting the poles of a battery, the
current ceases because an air gap intervenes and electricity does not
readily pass through air. Many substances besides air stop the flow of
electricity. If a strip of glass, rubber, mica, or paraffin is
introduced anywhere in a circuit, the current ceases. If a metal is
inserted in the gap, the current again flows. Substances which, like
an air gap, interfere with the flow of electricity are called
non-conductors, or, more commonly, insulators. Substances which, like
the earth, the human body, and all other moist objects, conduct
electricity are conductors. If the telephone and electric light wires
in our houses were not insulated by a covering of thread, or cloth, or
other non conducting material, the electricity would escape into
surrounding objects instead of flowing through the wire and producing
sound and light.

In our city streets, the overhead wires are supported on glass knobs
or are closely wrapped, in order to prevent the escape of electricity
through the poles to the ground. In order to have a steady, dependable
current, the wire carrying the current must be insulated.

Lack of insulation means not only the loss of current for practical
uses, but also serious consequences in the event of the crossing of
current-bearing wires. If two wires properly insulated touch each
other, the currents flow along their respective wires unaltered; if,
however, two uninsulated wires touch, some of the electricity flows
from one to the other. Heat is developed as a result